This invention relates to extrusion dies for apparata producing seamless thermoplastic tubings, single-walled and double-walled. Particularly, the present die is adapted for an apparatus producing pipes with a smooth inner wall and a corrugated outer wall.
Known dies of such a type for single walled pipes (for example, U.S. Pat. No. 3,907,961) include the following components:
an extrusion head having a central bore with an opening for receiving an extrudate of a thermoplastic material under pressure therefrom; PA1 a nozzle attached to said head and carrying an outer die lip on a free end of the nozzle; PA1 a mandrel placed in said bore in a substantially coaxial relation with and spaced from the nozzle to provide a thermoplastic conducting passage extending in the longitudinal direction thereof for the delivery of the extrudate to the delivery end thereof carrying an inner die lip defining with the outer die lip an annular extrusion orifice of diminishing interior dimensions, through which the extrudate can flow uninterruptedly and form the tubing.
In addition to the above components, known dies for double-walled pipes (for example, U.S. Pat. No. 4,510,013) include another opening for the second extrudate, and a second hollow mandrel placed within the first one in a substantially coaxial relation with and spaced from the first mandrel to provide the second thermoplastic conducting passage extending in the longitudinal direction thereof for the delivery of the second extrudate to the delivery end thereof carrying a second inner die lip defining with the second outer die lip the second annular extrusion orifice of diminishing interior dimensions, through which the second extrudate can flow uninterruptedly and form the inner wall.
Also, the known dies are provided with internal cooling which is particularly important for the double-walled corrugated pipes, wherein the outer wall is set by a corrugator, and the inner wall is set internally by an additional mandrel mounted coaxially with the hollow mandrel, downstream thereof, for urging the tubing outwardly and extracting heat from the latter by means of a plurality of channels cooling an outer surface of the additional mandrel receiving the tubing from the extrusion orifice, said channels being connected to a coolant discharge and a coolant supply located exteriorly of the tubing and communicating with said channels through the hollow mandrel.
The known dies have following disadvantages. They do not allow to vary the wall thickness of the tubing and to center the extrusion orifices. Also, the protruding second mandrel of the die for double-wall pipes lacks a support at its free end, the second extrudate having loss of heat in the second thermoplastic conducting passage.
Still another disadvantage of the known dies lies in having helical channels for coolant in the additional cooling mandrel, the lateral surface of the pipe being cooled helically. This effect causes stresses in the annular cross-sections of the pipe.
Finally, said channels constitute only one cooling circuit not allowing to control properly the heat exchange between the cooling mandrel and the inner surface of the pipe.